1. Field of the Invention
This invention is directed to novel carbon-sulfur composites which are suitable for utility as an electrode active material. In particular, the invention is directed to carbon-sulfur composite particles which are coated with a membrane of layers of polyelectrolytes, each successive layer having a charge opposite to the previous layer. The invention is also directed to a cathode containing the membrane coated carbon-sulfur composite and an electrochemical cell or battery containing the cathode. The invention is further directed to a lithium-sulfur battery containing the membrane coated carbon-sulfur composite cathode.
2. Discussion of the Background
In an ongoing effort to develop alternative vehicle energy power sources to the combustion engine, one area of development has been a plugin electric vehicle. To date much of the effort has been directed to lithium ion batteries as a power source for such vehicles. However, to become mainstream and to compete with the combustion engine in terms of cost and driving range, a significant improvement in the energy density of lithium ion batteries is necessary. The Holy Grail of post lithium ion research is to increase energy densities by utilization of conversion cathodes with high volumetric capacities such as sulfur or oxygen in combination with a pure metal anode. As an active cathode material, elemental sulfur can provide five times higher capacity than conventionally employed materials based on transition metal oxides or phosphates.
Although studies in lithium/sulfur battery date back to the early 1960's, the system has yet become commercially significant due to limited cycle life. Various problems have hindered the practical use of this highly attractive cathode including the insulating nature of sulfur which retards its reduction and poor electrode stability owing to the solubility in the electrolyte of the polysulfides generated during reduction of sulfur (Li2Sx, x=8, 6, 5 and 4). Over the last fifty years, methods for preventing migration of polysulfides have been intensively investigated by research teams worldwide. Significant advancements have been achieved by optimizing the electrolyte composition or replacing the liquid based electrolyte with polymeric electrolytes.
One approach to solving this problem is to restrain the polysulfides generated by constraint of the sulfur into metal organic frameworks or a conductive host such as porous carbon. However, this technique offers only a partial confinement to the polysulfide species, and capacity losses observed after 100 cycles are often too great to provide useful lifetime. In other approaches to further confine the highly polar polysulfide species, the surface of the carbon has been adjusted by functionalizing with inorganic oxides or polymers with the aim of providing an exterior coating to restrict migration of polysulfides.
In order to extend the lifetime beyond that offered by carbon/sulfur composites, the present inventors envision encapsulation of an active sulfur electrode material within thin membranes which would restrict the diffusion of polysulfides while allowing for diffusion of the lithium ion. Polyelectrolyte multilayer (PEML) assembly is known as a technique for producing thin fuel cell membranes and for the assembly of nanowires for lithium ion battery electrodes. Furthermore, graphite and carbon nanotubes have been functionalized with multilayer polymeric films.
PEML assembly was introduced by Decher et al. (Science 277, 1232(1997)) and involves the simple, sequential adsorption of oppositely charged polyions from a dilute solution onto a substrate. This tunable technique utilizes the complex formation between polyanions and polycations. With access to a wide range of polyion building blocks for the PEML capsule, this methodology gives access to a “Lego” set of membrane structures with infinite prospects for utility in the enhancement of the cycle life of lithium sulfur batteries. Since introduction by Decher et al., PEML type membranes have been employed in varying applications.
Hammond et al. (Soft Matter, 2007, 3, 804-816) provides a review of layer-by-layer (LbL) techniques and assemblies. Use of LbL to form a cathode containing LiCoO2 and carbon is described among other systems. Hammond states: “Using the right combination of materials, such as a polymer electrolyte and an active redox electrode material, the LbL technique improves the specific capacity and performance of battery electrodes by maximizing the reactive area.”
Sukhorukov et al. (Angew. Chem. Int. Ed. 2003, 42, 4471-4475) describes formation of inorganic/organic nanocomposite microcapsules. Capsule shell layers of poly(allylamine hydrochloride (PAH) and poly(styrene sulphonate) (PSS) over cores of Fe2O3, Y2O3 and calcium hydroxyapatite are described. Utility of the capsules to contain carbon/sulfur composites or use as electrode materials is not disclosed or suggested.
Kane et al. (Nano Letters, 2003, Vol. 3, No. 10, 1437-1440) describes preparing polymer multi-layers on the surfaces of graphite, carbon SWNT's and carbon MWNT's. Described polymer layer sequences included starting at the carbon surface, a hydrolyzed-poly(styrene-alt-maleic anhydride) (h-PSMA), polyethyleneimine (PEI), polyacrylic acid (PAA) and PEI. Layer coating of carbon/sulfur composites or use as electrode materials is not disclosed or suggested.
Donath et al. (Angew. Chem. Int. Ed. 1998, 37, No. 16, 2201-2205) describes micron-size polyelectrolyte shells of poly(sodium styrenesulfonate)(PSS) and poly(allylamine hydrochloride)(PAH). Hollow spheres are prepared by deposition of alternating oppositely charged polyelectrolyte layers from dilute solution onto melamine formaldehyde colloidal particles. After formation of the polyelectrolyte shell, the colloidal particles are decomposed and removed from the resulting hollow shells.
Cui et al. (ACSNANO, Vol. 5, No. 11, 2011, 9187-9193) describes a cathode prepared by drop casting a solution containing a mesoporous carbon/sulfur composite coated with commercially available solution of poly(3,4-ethylenedioxy-thiophene)/poly(styrene sulphonate) (PEDOT:PSS). Description of testing of the cathode in a cell with a lithium anode and lithium bis(trifluoromethanesulfonyl)imide in 1,3-dioxolane and 1,2-dimethoxyethane is disclosed. Cui does not disclose coating the mesoporous carbon/sulfur composite with alternating layers of oppositely charged polyelectrolytes.
Bruce et al. (Nature Materials Vol. 11, 2012, 19-29) review problems associated with development of high capacity Li—S batteries and describes technical advances toward addressing these problems. The design of porous composite cathodes containing sulfur which are capable of delivering electrons efficiently to the S as well as trapping the soluble polysulfides is described. Further, cathodes based on ordered nanostructured mesoporous carbon-sulfur composites are described as providing higher and more sustained, reversible capacities. A mesoporous carbon/sulfur composite with alternating layers of oppositely charged polyelectrolytes is not described nor suggested as a cathode material.
Wang et al. (Nano Letters, 2011, 11, 2644-2647) describe a graphene-sulfur composite as a cathode material for a Li—S cell. PEG-containing surfactant coated sulfur particles wrapped in grapheme oxide sheets containing carbon black are prepared and annealed to form a cathode of a coin cell with a Li anode and lithium bis(trifluoromethanesulfonyl)imide in 1,3-dioxolane and 1,2-dimethoxyethane as electrolyte. Coating of a carbon/sulfur composite with alternating layers of oppositely charged polyelectrolytes is not disclosed.
Son et al. (U.S. 2012/0315545) describes a lithium-sulfur battery having a hydrophilic polysulfide confinement layer placed between the cathode and the separator. The polysulfide confinement layer serves to minimize the amount of lithium polysulfide formed at the cathode which migrates from the cathode reaction area. A cathode constructed of sulfur, conductive material and binder applied to an aluminum collector is described. The polysulfide confinement layer is constructed by grafting PEG onto a porous PE or PP membrane, subjecting the grafted material to an oxygen plasma treatment and grafting a silane onto the PEG. Thus a porous hydrophilic membrane with a PEG polymer brush attached to the surface is formed. A mesoporous carbon/sulfur composite with alternating layers of oppositely charged polyelectrolytes is not described nor suggested as a cathode material.
Marinis et al. (U.S. 2011/0097623) describes batteries having electrodes which may have a plurality of protrusions and describes a nonporous electrolyte disposed on the electrode. Among structures suitable to form the electrolyte, is described a polyelectrolyte multilayer film which may be formed by a layer to layer deposition process. Marinis does not describe sulfur as a cathode material.
Naoi et al. (U.S. Pat. No. 5,792,575) describes a lithium-sulfur secondary battery containing a cathode composed of a sulfur compound, a highly basic polymer and a conductive material. Examples of the highly basic polymer include polyvinyl pyridine and polyvinyl pyrrolidone. The sulfur compound is a disulfide or polysulfide and the highly basic polymer is disclosed as trapping lithium thiorate formed during discharge within the cathode. Naoi does not disclose or suggest a coating of alternating layers of oppositely charged polyelectrolytes.
Schlenoff (U.S. Pat. No. 7,713,629) describes polyelectrolyte films of perfluorinated charged polymer layers which are useful in providing hydrophobic surfaces to articles such as carpet, shoes, metal surfaces. particles. Use as membranes for fuel cells, coatings for anti-friction surfaces, and coatings for electroluminescent materials is suggested. Schlenoff describes that to obtain good adhesion to a surface, a layer of polyethyleneimine may first be applied followed by layer by layer application of negative and positive polyelectrolyte. Poly(dimethyldiallyl ammonium chloride) and poly(styrenesulfonic acid) are listed as nonfluorinated polyelectrolytes. Schlenoff does not disclose or suggest coating of a carbon/sulfur composite with alternating layers of oppositely charged polyelectrolytes nor any utility of the hydrophobic fluorinated polyelectrolyte complex films as an electrode material.
Neudecker et al. (U.S. 2009/0181303) describes a multilayered thin film which is used to encapsulate an electrochemical device such as a thin film battery. Neudecker does not describe polyelectrolyte alternating layers and is not directed to electrode structure or lithium-sulfur batteries.
Jung et al. (U.S. 2004/0048164) describes design of an electrolyte system for a lithium-sulfur battery. The electrolyte system contains a mixture of dimethoxyethane, dioxolane and diglyme. The dimethoxyethane and diglyme dissolve polysulfide and the dioxolane is described as generating a protective polymer coating on the lithium surface during charge and discharge. The cathode may contain elemental sulfur, electrically conductive materials and a binder. Jung describes a separator as being multi-layers of polyethylene and poly propylene. Jung does not disclose or suggest a cathode containing a carbon/sulfur composite with alternating layers of oppositely charged polyelectrolytes.
Drzal et al. (U.S. 2010/0092809) describes films of exfoliated graphite nanoparticles which can be formed by layer by layer application of polyelectrolyte dispersions of the graphite nanoparticles. Utility of the electrically conductive flexible thin films in lithium ion storage batteries and as a low cost alternative to carbon nanotubes is suggested. Drzal does not describe a lithium sulfur battery nor is utility of the graphite nanoparticles films for the construction of a sulfur cathode disclosed or suggested.
Skotheim et al. (U.S. 2011/0165471) describes an anode for an electrochemical cell containing lithium and multiple protective layers which are ion conducting. Electrically conductive polymers, ionically conductive polymers, sulfonated polymers and hydrocarbon polymers are indicated as suitable. Layer-by layer application of oppositely charged polyelectrolytes is not described. Active sulfur containing cathodes are described as containing conductive fillers and binders. Skotheim does not describe or suggest carbon/sulfur composite with alternating layers of oppositely charged polyelectrolytes as a cathode material.
Chiang et al. (U.S. 2012/0244444) describes construction of a porous electrode, especially for a small battery. The porous sintered electrode is a ceramic material such as LiCoO2. Chiang describes that a polyelectrolyte multilayer film may be prepared by a layer-by-layer deposition process but does not describe a cathode having sulfur as an active component and does not describe a carbon/sulfur composite with alternating layers of oppositely charged polyelectrolytes as a cathode material.
None of these references discloses or suggests carbon/sulfur composites which are coated by LbL methods to obtain a membrane of alternating polyelectroyte layers of opposite charge as a coating.
The inventors are directing effort and resources to the study of materials useful to produce a battery of sufficient capacity and cycle lifetime to be competitive with and replace a combustion engine as a power source as well as other utilities requiring a high capacity, high cycle lifetime battery. In addition, a battery suitable for large scale intermittent energy storage will also be important for storage of green energy such as provided by wind and solar generation methods.
Therefore, an object of the present invention is to provide a sulfur composition which is suitable for utility as an electrode active material for a battery having high capacity and high cycle lifetime.
A second object of the invention is to provide a cathode containing sulfur as an active material which is suitable for utility in a battery having high capacity and high cycle lifetime.
A third object of the invention is to provide a lithium-sulfur battery which has sufficient capacity and lifetime to be a viable energy source for a vehicle.